Non-volatile memories are devices that can retain stored information without the use of external power sources. They are usually insulated gate field effect transistors modified so that electrical charge can be stored in the gate dielectric and thus cause a threshold change in the transistor. Such devices can be separated into two broad classes, depending on the charge storage mechanism. The first class contains devices where charges are stored in deep energy states at or near the interface between two gate dielectrics. The best known devices of this type are the MNOS (metal-silicon nitride-silicon dioxide-silicon) and the MAOS (metal-aluminum oxide-silicon dioxide-silicon) devices. Also in this class is a new and improved device disclosed in my copending application Ser. No. 645,150, filed Dec. 30, 1975, and entitled "Non-volatile Field Effect Semiconductor Memory Structure Utilizing Implanted Ions to Induce Trapping States," which is assigned to the present assignee. For devices of this first class, the conduction mechanism used to transfer charges to the storage sites from the substrate is known as "tunneling." Tunneling takes place upon the application of an appropriate control voltage pulse at the gate electrode. Tunneling is also used to remove the charges from the storage sites to their original positions upon the application of a control voltage pulse of a polarity opposite to that used for the transfer and storage of the charges initially.
The other class of these memory devices contains structures in which the charge storage takes place in a floating electrode buried in the gate dielectric. The best known device of this second class is the FAMOS (floating gate-avalanche injection-MOS) type. In a FAMOS device, charges are transferred into a floating gate by the avalanche injection of electrons from a reverse biased drain junction. A more detailed discussion of both the MNOS and the FAMOS structures is contained in a publication by Andrew C. Tickle and Frank M. Wanlass, entitled "Electrically Alterable Nonvolatile Semiconductor Memories," Session 4, 1972 Wescon Technical Papers.